1. Field of the Invention
The present invention relates to a method for testing the strength of ceramic engine-valves and a testing device for the same.
2. Description of the Related Art
A valve for an engine is exposed to high temperature combustion gas and exhaust gas in a cylinder and has to tolerate the internal pressure, the inertial force, and the impactive force in the cylinder as well, so that heat resisting alloys have been conventionally used for forming it. Recently, ceramic such as nitriding silicon that is lighter in weight and more excellent in heat resistance than the heat resisting alloy is tried to use for forming the valves instead of heat resisting alloys. The engine-valve is a reciprocating part at the high speed; when it is made lighter by forming it of the ceramic, it has advantages such as decrease in mechanical failures, improving in engine revolutions, and reduction in engine noises.
However, ceramic has poor ductility compared with metals and when excessive stress is applied to a region of minute defects, a crack originated from the defect is expanded to destruction. Since the lager the defect is, the larger the localized concentrated stress at end portions of the defect is produced, the allowable defect size becomes smaller in the portion to which the larger the stress is applied.
As for the portion to which a small stress is applied, it is possible to guarantee the reliability in the strength of the valve by confirming no large defect by means of a non-destructive defect test such as ultrasonic flaw detection and a penetrating X-ray method; however the non-destructive defect test is insufficient for small defect sizes in portions to which a large stress is applied such as a cotter-groove portion disposed in the vicinity of the end portion of a stem portion and a cotter-fastening portion fastened by a cotter disposed closer to a head portion than the cotter-groove portion because the allowable defect size is small therein.
Therefore, as for the portion to which a large stress is applied as described above, the so-called proof test confirming that no destruction occurs under a predetermined stress by practical carrying out of a load test to produce the predetermined stress in the portion is proposed as a substitution for the non-destructive defect test.
As the proof test, a method such as a tensile test that a cotter is fitted to the stem portion just like in the practical use to apply axial forces to the head portion and the cotter in directions opposite each other and a method (a rotating bending test) that the head portion and the end portion of the stem portion are rotatably held by holding jigs so as to apply a load in a direction roughly perpendicular to the stem portion while rotating the valve is proposed (see Japanese Unexamined Patent Publication No. 3-13842).
However, the tensile test method is improper for inspecting manufactured articles because fitting the parts and setting the valve on the testing device take a long time. The conventional rotating bending test also involves a problem that when a stress is applied to test the cotter-fastening portion to which the maximum stress is practically applied, the same amount of stress or more is produced in the cotter-groove portion to carry out an excessive test relative to the cotter-groove portion after all.
In the rotating bending test, inserting the end portion of the stem portion into the holding jig is the only method for holding it; the amount of clearance between the holding jig and the stem portion is needed; the stem portion is inclined in the holding jig because the holding span is short, producing variations in the stress applied during rotation. Furthermore, in the rotating bending test, the head portion is held by screws, etc., for setting it in the testing device, so that a long time is needed and there is a danger of damaging the valve face of the head portion as well.
In view of these conventional problems, the present invention has been made. Accordingly, it is an object of the present invention to provide a test method on the strength of ceramic engine-valves, in which when the strength of a cotter-fastening portion and a cotter-groove portion of the valve, to which large amounts of stress are applied when the valve is used, are tested (rotating bending test), unnecessary stress is not applied to portions other than these places to be tested; the applied stress is not changed during testing; a long time is not taken to set the valve on a testing device; and the valve face of a head portion cannot be damaged. It is another object of the present invention to provide a strength-testing device enabling to carry out such the test preferably.
In accordance with a first aspect of the present invention, there is provided a method for testing the strength of a ceramic engine-valve (a first method for testing the strength) comprising an umbrella-shaped head portion and a bar-shaped stem portion, the method comprising the steps of: fastening and holding only a cotter-fastening portion to be fastened by a cotter, disposed closer to the head portion than a cotter-groove portion disposed in the vicinity of the end portion of the stem portion for fitting the cotter thereto, to one end portion of a main spindle connected to a rotational driving system coaxially with the main spindle; rotatably holding the stem portion at a position separated from the fastening-holding portion by a predetermined span; and applying a load in a direction perpendicular to the stem portion axis at the position on the stem portion separated from the fastening-holding portion by the predetermined span while the valve is rotated about the axial line thereof by rotating the main spindle so as to test the strength of the cotter-fastening portion.
In accordance with a second aspect of the present invention, there is provided a method for testing the strength of a ceramic engine-valve (a second method for testing the strength) comprising an umbrella-shaped head portion and a bar-shaped stem portion, the method comprising the steps of: fastening and holding only a portion closer to the end portion of the stem portion than a cotter-groove portion for fitting a cotter thereto disposed in the vicinity of the end portion of the stem portion to one end portion of a main spindle connected to a rotational driving system coaxially with the main spindle; rotatably holding the stem portion at a position separated from the fastening-holding portion by a predetermined span; and applying a load in a direction perpendicular to the stem portion axis at the position on the stem portion separated from the fastening-holding portion by the predetermined span while the valve is rotated about the axial line thereof by rotating the main spindle so as to test the strength of the cotter-groove portion.
In accordance with a third aspect of the present invention, there is provided a device for testing the strength of ceramic engine-valves (a first strength-testing device), comprising: a main spindle held rotatably about the axis thereof, one end thereof having a tapered hole and the other end connected to a rotational driving system via a universal joint; and means for applying a load to applying a load in a direction perpendicular to the axis of a stem portion of the ceramic engine-valve while rotatably holding the stem portion which is fastened and held in the vicinity of the end portion of the stem portion by a collet inserted into the tapered hole of said main spindle and an inner collet fitted inside the collet, wherein the collet has a tapered conical surface in the outer periphery thereof corresponding to that of the tapered hole of the main spindle and is also elastically deformable in the inner-diameter-decreasing direction by applying a radial force from the outer periphery thereof, and wherein the inner collet has an inner peripheral shape for contacting and holding only a cotter-fastening portion of the stem portion of the valve and is also elastically deformable in the inner-diameter-decreasing direction by applying a radial force from the outer periphery thereof.
In accordance with a fourth aspect of the present invention, there is provided a device for testing the strength of ceramic engine-valves (a second strength-testing device), comprising: a main spindle held rotatably about the axis thereof, one end thereof having a tapered hole and the other end connected to a rotational driving system via a universal joint; and means for applying a load to apply a load in a direction perpendicular to the axis of a stem portion of the ceramic engine-valve while rotatably holding the stem portion which is fastened and held in the vicinity of the end portion of the stem portion by a collet inserted into the tapered hole of the main spindle and an inner collet fitted inside the collet, wherein the collet has a tapered conical surface in the outer periphery thereof corresponding to that of the tapered hole of the main spindle and is also elastically deformable in the inner-diameter-decreasing direction by applying a radial force from the outer periphery thereof, and wherein the inner collet has an inner peripheral shape for contacting and holding only a portion closer to the end portion of the stem portion than a cotter groove disposed in the stem portion of the valve and is also elastically deformable in the innerdiameter-decreasing direction by applying a radial force from the outer periphery thereof.